1. Field of the Invention
The present invention relates to a method for detecting a fault on transmission lines, and more particularly to a method for precisely detecting a fault on transmission lines by using harmonics and a state transition diagram of an electric power system, in which apparent impedance having steady state characteristic and harmonics having transient state characteristic are used as input parameters of the state transition diagram so as to presume a state of the electric power system, thereby detecting the fault on the transmission lines even if the transmission lines are subject to unstable voltage and overload.
2. Description of the Prior Art
Distance relays use apparent impedance calculated by using voltage and current applied to lines. When a ground fault or a short circuit fault occurs on the lines, a value of apparent impedance of a distance relay is reduced. If apparent impedance having a reduced value enters into a predetermined zone, the fault is determined. FIG. 1 shows a path of apparent impedance when the fault occurs. After the fault occurs, a location of apparent impedance is varied depending on a distance between the distance relay and a fault position. Thus, it is possible to determine the fault and the fault position by using the location of apparent impedance.
Hereinafter, a malfunction of the distance relay will be described. If load of an electric power system is increased on transmission lines, voltage is reduced, current is increased, and apparent impedance calculated by the distance relay is reduced. When load is continuously increased, apparent impedance may enter into a zone. If apparent impedance enters into the zone due to increased load rather than the fault, which is called “load encroachment”, the distance relay is malfunctioned. In addition, if voltage is reduced and current is increased due to an unstable state of voltage, apparent impedance may enter into the zone.
Such malfunction of the distance relay is frequently created at an outer portion of the zone, that is, at a third zone shown in FIG. 1.
Conventionally, in order to detect the fault of transmission lines, a trap and a tuner having a radio frequency band are installed at both sides of a transmission line. Then, the fault of the transmission line is detected by using an inhibitatory signal and an operating signal extracted from radio frequency component. However, not only is an additional hardware required to extract the radio frequency component, but also it is difficult to reliably detect the fault by using the radio frequency component.